This invention relates to a method of polishing materials such as metals, glass, ceramics and plastics and to an agent for use for processing such materials. In particular, this invention relates to such an agent suitable for use in texturing substrates made of such materials for producing magnetic hard disks.
For producing magnetic disks, glass substrates and aluminum plates with a NiP-plated surface are widely being used. Such a magnetic disk is produced by first having its surface mirror-finished and textured and then having a magnetic layer formed thereon, say, by sputtering. The texturing is a production process by which concentric circular texture lines are created on the surface of the magnetic disk substrate for improving the magnetic characteristic of the magnetic disk as a magnetic recording medium by providing it with magnetic non-isotropy and also to prevent the adsorption between the magnetic head and the magnetic disk surface when the hard disk drive is not operating.
The recording capacity of a magnetic disk as a magnetic recording medium can be increased by reducing the distance between the magnetic head and the magnetic disk surface. Unless the texture lines on the surface of the magnetic disk substrate are formed uniformly, the magnetic head will strike abnormal protrusions on the magnetic disk surface. Unless the texture lines are finely formed, furthermore, the magnetic head tends to become adsorbed to the magnetic disk surface and hence its separation from the disk surface cannot be reduced and the recording capacity cannot be increased. In other words, the magnetic disk surface must be textured such that lines are finely produced and at a high density without leaving any abnormal protrusions on the surface in order to increase the recording capacity of the magnetic disk.
The texturing of a magnetic disk substrate is carried out by supplying a processing agent obtained by dispersing abrasive particles in water of an aqueous solution to the surface of the substrate and pressing and causing to run a woven or unwoven polishing cloth or a polishing tape against the substrate surface.
As for the texturing processing agent, use is commonly made of a slurry obtained by dispersing about 0.01-5 weight % of abrasive particles in an aqueous solution containing a glycol compound surfactant such as polyethylene glycol and polypropylene glycol in an amount of about 1-25 weight % in order to have the abrasive particles dispersed uniformly within the liquid and to prevent the precipitation of the abrasive particles while the agent is being kept in storage.
As for the abrasive particles, monocrystalline or polycrystalline diamond particles are used. Diamond particles are preferable because they are shaped regularly and variations are small in their sizes and shapes. Furthermore, they are hard and highly resistant against chemicals and heat. In particular, polycrystalline diamond particles are more widely used than monocrystalline diamond for texturing because they are round particles without sharp edges.
The agent for texturing is obtained by adding abrasive particle to an aqueous solution containing a surfactant, decomposing the secondary particles (agglomerated abrasive particles) in the liquid, say, by means of a ultrasound dispersing apparatus, after sufficiently stirring the mixture, and thereby causing the abrasive particles to be dispersed inside the liquid in the form of primary particles. A filter may preferably be further used to eliminate the secondary particles which may have agglomerated inside the liquid through precipitation. In summary, it was commonly believed that the surface of a magnetic disk substrate is damaged by the agglomerated secondary particles and this was why attempts were being made to decompose the secondary particles inside the liquid through ultrasonic waves or the like to disperse the abrasive particles inside the liquid in the form of primary particles.
Japanese Patent Publication Tokkai 11-138424, for example, disclosed an agent for texturing obtained by dispersing polycrystalline diamond particles with average diameter of 0.05-5 xcexcm in the form of primary particles and it is reported that the surface of a magnetic disk substrate can be textured such that the average surface roughness Ra is 4.0 xc3x85 and there are hardly any abnormal protrusions which are visible on the surface.
With the recent increase in the speed of data processing by computers, however, the memory capacity of magnetic disks serving as magnetic recording media is required to be increased. Thus, further improvements in the texturing technology are required such that the surface of a magnetic disk substrate can be textured at a higher density without any abnormal protrusion left on the surface.
It is therefore an object of this invention in view of such demands for improved texturing technology to provide a method by which one or both of the surfaces of a substrate of a magnetic disk can be textured more uniformly and more finely than previously possible.
It is another object of this invention to provide an agent which may be used in such an improved method of texturing.
An agent for texturing embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising an aqueous solution containing glycol compounds in an amount of about 1-25 weight % serving as a surfactant and agglomerated polycrystalline diamond particles with average diameter of 0.01-2 xcexcm comprised of polycrystalline diamond particles with average diameter of 1-20 nm in an amount of 0.001-3 weight % dispersed in this aqueous solution. Examples of surfactant which may be used according to this invention include glycol compounds such as polyethylene glycol and polypropylene glycol. Polyoxyethylene alkyl phenyl ether sodium phosphate may be appropriately used for the purpose.
For texturing a substrate such as a glass or aluminum substrate for a magnetic disk by a method embodying this invention, the substrate is rotated, the agent as described above is supplied to its surface and a woven, non-woven, foam or planted tape is pressed and run against it. It is to be noted that it is agglomerated polycrystalline diamond particles (secondary particles) which are agglomerations of primary polycrystalline diamond particles and have an average diameter within a specified range that are used as abrasive particles. It is further to be noted that these primary polycrystalline diamond particles which are agglomerated to form the abrasive particles of this invention have extremely small diameters, have little variations in size and shape, are hard, are highly resistant against chemicals and heat and are of a circular shape without sharp edges. If such primary polycrystalline diamond particles were directly used (that is, not in an agglomerated form) for the texturing process, they would easily pass through the space between the fibers of the woven, non-woven or planted tape which may be used for the polishing. If a foam tape were being used, such small diamond particles would easily go inside the bubble spaces on the surface of the tape. In other words, it would not be possible to dependably keep these particles on the tape surface to be pressed against the substrate surface to be textured.
Secondary polycrystalline diamond particles, by contrast, are agglomerations of a large number of extremely small round particles without sharp edges, having around themselves many extremely small protrusions formed by these primary particles. Since these extremely small protrusions come into contact with the surface of the substrate, the agglomerated polycrystalline diamond particles can remain on the surface of the tape more easily during the texturing process and can be pressed against the substrate surface more reliably. Because these secondary particles are agglomerations of extremely smaller primary particles, furthermore, this means that more particles process unit areas of the target substrate surface than if particles with the same diameters as secondary particles were used. As a result, the substrate surface can be textured extremely finely and uniformly if it is processed by using an agent according to this invention, containing agglomerated polycrystalline particles.